This invention relates to fiber optic sensing systems wherein a sensor is interposed in the system between a light source and a receiver and wherein the sensor is responsive to a parameter to alter the intensity of the light passing therethrough.
Fiber optic sensing systems generally utilize a sensor interconnected by optical waveguides between a light source and a receiver. The sensor includes one or more transducers that respond to an external parameter, for example ambient pressure, to vary the intensity of an optical signal that is generated by the light source for propagation through the system. An example of a typical application would be one in which a transducer attenuates the intensity of the light signal in inverse proportion of the magnitude of the parameter. The receiver defects the change in intensity of the signal. Suitable processing circuitry then produces an indication of the magnitude of the parameter.
The intensity of the light that propagates through the system is not varied solely by the transducer in the sensor. That is, numerous other factors tend to vary the intensity of the propagating light. The most significant of these factors include: variations in light source intensity; connector losses; and, fiber path losses. These factors are inherent in the construction of the system and their effects will vary with the age of the system and changes in the environment surrounding the system. For accurate determination of the intensity variation caused solely by the transducer(s) it is necessary to compensate for the additional intensity variations caused by these other factors.
In order to provide proper compensation for the intensity variations that are caused by factors other than the transducer, it has been recognized in the prior art that the receiver should be provided with a reference signal that propagates along an optical path that is separate from the path of the signal whose intensity is varied by the transducer (the latter signal hereinafter referred to as the "sensed signal"). This separate optical path, which allows the reference signal to be distinguished from the sensed signal, is defined by an optical waveguide that is constructed to be substantially identical to the sensed signal optical waveguide except that it bypasses the transducer. That is, the length, connectors and other elements of the reference waveguide match, as nearly as possible, those corresponding elements of the sensed signal waveguide. The separate optical path for the reference signal also necessitates the use of matched separate receiver channels with their associated connectors. By matching the waveguide elements, both the reference and sensed signals should undergo substantially identical intensity variations caused by the above-described factors. As noted, the reference signal bypasses the transducer, thus its intensity is not further varied by the transducer. Normalizing the sensed signal intensity (i.e., taking a simple ratio of the sensed signal to the reference signal) will yield the dimensionless value of intensity variation caused by the transducer--irrespective of the intensity variations caused by the other factors.
A problem with the just-described compensation method is that with the present state of the art it is extremely difficult to precisely match optical paths for duplicating all intensity-varying factors. To give an example, for a typical optical signal path, the decoupling and remating of a single connector may cause a change in the transmitted intensity by as much as 50%. Where it is desirable to employ a transducer having an accuracy on the order of 0.1% it can be appreciated that when attempting to match waveguides that employ several connectors, a rather significant opportunity for error is introduced each time a connection is decoupled and remated. Furthermore, short radius bends (less than 5 centimeter bend radius) can introduce attenuation on the order of several percent in an optical waveguide. Thus, if the reference signal waveguide and the sensed signal waveguide are not properly matched, or if after matching, one of the waveguides is bent, disconnected and remated, or otherwise moved relative to the other waveguide, then the resulting normalization of the sensed signal intensity will not accurately reflect the intensity variation caused solely by the transducer.